In silver halide based color photography, a typical photographic element contains multiple layers of light-sensitive photographic silver halide emulsions coated on a support with one or more of these layers being spectrally sensitized to each of blue light, green light and red light. The blue, green, and red light-sensitive layers typically contain yellow, magenta, and cyan dye-forming couplers, respectively. After exposure to light, color development is accomplished by immersing the exposed material in an aqueous alkali solution containing an aromatic primary amine color developing agent. The dye-forming couplers are selected so as to react with the oxidized color developing agent to provide yellow, magenta and cyan dyes in the so called subtractive color process to reproduce their complementary colors, blue, green and red as in the original image.
The important features for selecting the dye-forming coupler include, efficient reaction with oxidized color developing agent, thus minimizing the necessary amounts of coupler and silver halide in the photographic element; the formation of dyes with hues appropriate for the photographic use of interest, for color photographic paper applications this requires that dyes have low unwanted side absorption leading to good color reproduction in the photographic print; minimization of image dye loss contributing to improved image permanence under both ambient illumination and conventional storage conditions; and in addition the selected dye-forming coupler must exhibit good solubility in coupler solvents, provide good dispersibility in gelatin and remain stable during handling and manipulation for maximum efficiency in manufacturing processes.
In recent years, a great deal of study has been conducted to improve dye-forming couplers for silver halide photosensitive materials in terms of improved color reproducibility and image dye stability. However, further improvements are needed, particularly in the area of cyan couplers. In general, cyan dyes are formed from naphthols and phenols as described, for example, in U.S. Pat. Nos. 2,367,351, 2,423,730, 2,474,293, 2,772,161, 2,772,162, 2,895,826, 2,920,961, 3,002,836, 3,466,622, 3,476,563, 3,552,962, 3,758,308, 3,779,763, 3,839,044, 3,880,661, 3,998,642, 4,333,999, 4,990,436, 4,960,685, and 5,476,757; in French patents 1,478,188 and 1,479,043; and in British patent 2,070,000. These types of couplers can be used either by being incorporated in the photographic silver halide emulsion layers or externally in the processing baths. In the former case the couplers must have ballast substituents built into the molecule to prevent the couplers from migrating from one layer into another. Although these couplers have been used extensively in color photographic film and paper products, the dyes derived from them still suffer from poor stability to heat, humidity or light, low coupling efficiency or optical density, and in particular from undesirable blue and green absorptions which cause considerable reduction in color reproduction and color saturation.
Cyan couplers which have been recently proposed to overcome some of these problems are 2,5-diacylaminophenols containing a sulfone, sulfonamido or sulfate moiety in the ballasts at the 5-position, as disclosed in U.S. Pat. Nos. 4,609,619, 4,775,616, 4,849,328, 5,008,180, 5,045,442, and 5,183,729; and Japanese patent applications JP02035450 A2, JP01253742 A2, JP04163448 A2, JP04212152 A2, and JP05204110 A2. Even though cyan image dyes formed from these couplers show improved stability to heat and humidity, enhanced optical density and resistance to reduction by ferrous ions in the bleach bath, the dye absorption maxima (.lambda.max) are too bathochromically shifted (that is, shifted to the red end of the visible spectrum) and the absorption spectra are too broad with considerable amounts of undesirable blue and green absorptions. Thus, these couplers are not practical for use in color papers.
The hue of a dye is a function of both the shape and the position of its spectral absorption band. Traditionally, the cyan dyes used in color photographic papers have had nearly symmetrical absorption bands centered in the region of 620 to 680 nm, preferably 630 to 660 nm, and more preferably 635 to 655 nm. Such dyes have rather large amounts of unwanted absorption in the green and blue regions of the spectrum.
More desirable would be a dye whose absorption band is asymmetrical in nature and biased towards the green region, that is, with a steep slope on the short wavelength side. Such a dye would suitably peak at a shorter wavelength than a dye with symmetrical absorption band, but the exact position of the desired peak depends on several factors including the degree of asymmetry and the shapes and positions of the absorption bands of the magenta and yellow dyes with which it is associated.
Recently, Lau et. al. in U.S. Pat. No. 5,686,235 describe a particular class of cyan dye-forming coupler that has been shown to improve thermal stability and hue, particularly, with decreased absorption in side bands and an absorption band that is asymmetrical in nature. However, it has been found that dispersions of these couplers are difficult to prepare free of crystalline material, and are not stable with time in cold storage. Other related patents are U.S. Pat. Nos. 5,047,314, 5,047,315, 5,057,408, and 5,162,197.
Large-scale manufacturing of photographic materials can be severely hindered when crystalline material is present in dispersions and coating melts of such dispersions. This can lead to difficulty in manufacturing by plugging filters and causing defects in coatings of photographic materials. It is therefore desirable to use dispersions which have few, if any, crystals and are stable in cold storage from the time of preparation until the time of use.
This invention relates to so-called "NB couplers" which are defined more fully hereinafter. It has been found that preparing substantially crystal free dispersions of these "NB couplers" can be difficult. It appears that the property of these couplers that enables the dye formed by them to shift hue may at the same time be responsible for difficulties in the formation of unwanted crystals. Appropriate selection of a coupler solvent can reduce the amount of crystals to an acceptable level. However, it has been found that some "NB couplers", particularly those with high melting points, can fail to disperse in these preferred solvents as cleanly as couplers of lower melting points.
The use of various high boiling coupler solvents is disclosed in the following U.S. patents: U.S. Pat. No. 5,726,003, U.S. Pat. No. 5,047,315, U.S. Pat. No. 5,057,408, U.S. Pat. No. 5,356,768, U.S. Pat. No. 4,882,267, U.S. Pat. No. 4,767,697, U.S. Pat. No. 4,217,410, and U.S. Pat. No. 4,840,878. Experimental work contained in this specification has shown that a number of these known solvents may be employed to reduce the amount of crystals formed, but these solvents also result in decreased coupler reactivity and increased unwanted green light absorption.
The problem to be solved is to provide a photographic element and process in which a dispersion containing an "NB coupler" exhibits reduced crystal formation while maintaining satisfactory levels of reactivity and unwanted green light absorption.